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Abstract

A comparative study is carried out on two spectroscopic techniques employed to detect ultrafast absorption changes in the mid-infrared spectral range, namely direct multichannel detection via HgCdTe (MCT) photodiode arrays and the newly established technique of chirped-pulse up-conversion (CPU). Whereas both methods are meanwhile individually used in a routine manner, we directly juxtapose their applicability in femtosecond pump-probe experiments based on 1 kHz shot-to-shot data acquisition. Additionally, we examine different phase-matching conditions in the CPU scheme for a given mid-infrared spectrum, thereby simultaneously detecting signals which are separated by more than 200 cm−1.

Transient absorption of Co4(CO)12 under 270 nm UV excitation: data is obtained via CPU (left) and multichannel MCT detection (right), respectively. For each time step 1000 pump-probe pairs are consecutively recorded with each detection method. No data interpolation was used in both graphics.

Absorption-change noise level of a single data point at 2039 cm−1 when detecting ΔAbs = 0 (left panel, no sample) and ΔAbs ≈ 2 mOD (right panel, sample and pump-interaction): standard error versus the number of considered background-corrected pump-probe pairs using MCT detection (blue), CPU (red) or CPU with an additional binning of five adjacent data points (green). Noise levels resulting after normalization of individual spectra with their integrated spectral intensity are shown for CPU detection (black) and CPU detection with an additional binning of five adjacent data points (gray). Note that these graphs have a double-logarithmic scale.

Absorption-change standard error as a function of the spectral position: data obtained via MCT (blue) and CPU detection (red) after averaging 25000 consecutive pump-probe pairs when detecting ΔAbs = 0 (solid lines, no sample) and a molecular signal (dashed lines, sample and pump-interaction, confer transient absorption spectra in Fig. 4); the CPU curves with binned data points or with normalized spectra, or with both, are not shown for clarity; the corresponding CPU spectra, with sample (light red) and without sample (light gray), are shown in the background.